Lubricants (also referred to as lubricant compositions, lubricating oils or lubricating oil compositions), such as those used in a variety of machinery, are susceptible to oxidative deterioration during storage, transportation and usage, particularly when such lubricants are exposed to high temperatures and iron catalytic environments, which greatly promote their oxidation. This oxidation, if not controlled, contributes to the formation of corrosive acidic products, sludge, varnishes, resins and other oil-insoluble products, and may lead to a loss of designated physical and tribological properties of the lubricants. These oxidation products may lead to the formation of harmful deposits on critical engine parts, such as the pistons, piston liners, valves and valve lifters. It is therefore a common practice to include deposit-control and antioxidant additives in lubricants to prevent, at least to some extent, oxidation so as to extend their useful life.
“Chemistry and Technology of Lubricants” (second edition) edited by Mortier and Orszulik (ISBN 0 7514 0246 X) describes the inhibition of oxidative degradation of lubricants (section 4.4) and mention radical scavengers as one additive type suitable for this purpose. Among the radical scavengers are mentioned sterically-hindered phenols, exemplified as phenols that are substituted in the 2 and 6 positions with tertiary alkyl groups, the most common of which is the tertiary butyl group. It is stated that optimum protection is achieved when both 2 and 6 positions of the phenol are substituted with tertiary butyl groups and that replacement of a tertiary butyl group by a methyl group in the ortho position reduces the antioxidant activity considerably.
There is interest in providing alternative phenolic lubricants that are derivable from low-cost, widely-available and renewable raw materials. However, these are not indicated by Mortier and Orszulik.
Patil et al describe cashew nut shell liquid as a potential raw material that could be used for the preparation of oxidation inhibitors. They mention cardanol but note that a major drawback in its potential use as an additive is unsaturation in the linear chain. They discuss elimination of the double bonds by hydrogenation, but note the 15-carbon long chain will have poor thermal stability. They describe use of hydrogenated cardanol as an antioxidant in mineral oil, specifically electrical insulating petroleum (or transformed) oils.
UK Patent 626,251 describes use of meta pentadecenyl phenol, in straight chain form (a constituent of cardanol), as an additive to solubilise a metal detergent in an internal combustion engine lubricant. It does not mention the phenol as having anti-oxidancy properties, and notes that oxidation stability may be improved by blending with other additives.
UK Patent 633,188 describes use of minor propositions of distilled cardanol (chiefly metapentadecenyl phenol), in combination with a sulphurised mixture of alkenes, to improve the piston cleanliness performance of crankcase lubricants for automotive engines.